In general, an image sensor is a semiconductor device for converting optical images into electric signals and is mainly classified as a charge coupled device (CCD) image sensor or a CMOS image sensor.
However, a CCD has a complicated driving manner, high power consumption, and requires a multi-step photo process, which makes the manufacturing process thereof complicated.
For this reason, a CMOS image sensor has recently been spotlighted as the next-generation image sensor capable of overcoming the defects of the charge coupled device.
A CMOS image sensor is a device employing a switching mode to sequentially detect an output of photodiodes by providing MOS transistors corresponding to each unit pixel in conjunction with peripheral devices, such as a control circuit and a signal processing circuit.
That is, a CMOS image sensor with a photodiode and a MOS transistor within each pixel sequentially detects the electric signals of each unit pixel in a switching scheme to realize an image.
Since a CMOS image sensor is manufactured by utilizing CMOS technology, it has the advantage of relatively low power consumption. In addition, since a smaller number of photolithography steps is required, the manufacturing process of a CMOS image sensor can be simplified.
Further, since a control circuit, a signal processing circuit, an analog/digital converting circuit, and the like can be integrated on a single CMOS image sensor chip, the CMOS image sensor can minimize the size of a product.
Accordingly, a CMOS image sensor is widely used in various applications including digital still cameras, and digital video cameras.
CMOS image sensors are classified as a 3T type CMOS image sensor, a 4T type CMOS image sensor, or a 5T type CMOS image sensor according to the number of transistors formed in each unit pixel. The 3T type CMOS image sensor includes one photodiode and three transistors, and the 4T type CMOS image sensor includes one photodiode and four transistors.
FIG. 1 is an equivalent circuit diagram of a conventional 4T type CMOS image sensor, and FIG. 2 is a layout illustrating a unit pixel of the 4T type CMOS image sensor.
As illustrated in FIGS. 1 and 2, the unit pixel of the CMOS image sensor includes a photodiode 10 and four transistors. In particular, the unit pixel includes a photodiode 10 for receiving light and generating electrons formed at the wide region of the active area; a transfer transistor 20 for transferring electrons collected at the photodiode (PD) 10 to a floating diffusion (FD) region; a reset transistor 30 for setting electric potential at the floating diffusion (FD) region to a desired value and for exhausting electric potential to reset the floating diffusion (FD) region; a source follow transistor 40 functioning as a source follow buffer amplifier; and a select transistor 50 functioning as a switch for addressing.
Furthermore, as shown in FIG. 1, a load transistor 60 is formed at an output terminal (Vout) of each unit pixel 100 to read an output signal.
Referring to FIG. 1, Tx is a gate voltage applied to the transfer transistor 20, Rx is a gate voltage applied to the reset transistor 30, Dx is a gate voltage applied to the source follow transistor 40, and Sx is a gate voltage applied to the select transistor 50.
FIG. 3 is a cross-sectional view of the CMOS image sensor taken along the line II-II′ illustrated in FIG. 2.
Referring to FIG. 3, the CMOS image sensor includes an isolation layer 62 formed at an isolation region of a semiconductor substrate 61 on which the active area and the isolation region are defined; a gate electrode 64 formed on a predetermined area of the active area of the semiconductor substrate 61 isolated by the isolation layer 62 with a gate insulating layer 63 formed therebetween; a photodiode region 65 formed in an upper portion of the semiconductor substrate 61 at one side of the gate electrode 64; a floating diffusion region 66 formed in an upper portion of the semiconductor substrate 61 at the other side of the gate electrode 64; and an insulating layer sidewall 67 formed at both sides of the gate electrode 64.
FIG. 4 illustrates the operation of the transfer transistor shown in FIG. 3.
Referring to FIG. 4, the amount of responsive light may be determined by means of the capacitance of the photodiode (PD) region 65 and the capacitance of the floating diffusion (PD) region 66.
When the amount of an incident light through the photodiode (PD) region 65 is large enough, the floating diffusion (FD) region 66 can saturate and no more reaction proceeds. When the amount of the incident light is too small, the amount of the generated electrons (e) is too small and a sufficient reaction does not occur.